Change value of wxHELP to avoid clash with wxSTAY_ON_TOP.

Change value of wxHELP to avoid clash with wxSTAY_ON_TOP.

Using wxSTAY_ON_TOP with wxMessageDialog resulted in "Help" button appearing
since r68537 because these two constants had the same numeric value.

Avoid this by changing wxHELP value to be the same as wxCLOSE_BOX instead: the
latter can't be used with wxMessageDialog so this should solve the problem,
even if there are still conflicts. Also adjust the other button constants
values to account for wxHELP change.

Closes #13433.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69842 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775